In order to increase capacity and coverage for future 5G communication systems massive Multiple Input Multiple Output (MIMO) technology, small cells as well as more spectrum are some of the most promising means. Antenna array systems will be a ubiquitous component in such forthcoming 5G systems as a means of improving capacity for the presently used low GHz frequencies but also for ensuring sufficient coverage as the operating frequency extends to the mmW range. Antenna arrays may be used for simultaneously transmitting or receiving multiple layers, e.g. through multiple beams in the special case of line-of-sight (LOS) communication or more generally through the concept of Multi-User MIMO (MU-MIMO).
Uplink transmissions in cellular systems are typically controlled by an access node, in the following exemplified by an eNB. Based on, e.g., communicated transmission needs, channel properties and some measure of fairness, the eNB allocates the resources of a physical uplink shared channel (PUSCH) to the competing transmitter devices, in the following exemplified by user equipment (UEs). In case of a high load the eNB may choose to schedule a MU-MIMO transmission such that multiple UEs transmit in the same time-frequency resource. Since such scheduling increases noise, e.g., from interference from other transmissions (or layers) using the same time frequency resource, typically only transmitters with suitable transmission properties (i.e., weakly correlated channels) are selected. Non-orthogonal scheduling techniques have been proposed wherein a strong received signal is combined with a weak received signal whereby the strong signal is (presumably correctly) decoded and then recoded and subtracted from the jointly received signal after which the weak signal may be decoded.
In an antenna array system (AAS) receiver architecture with a common local oscillator generation, signals from all antenna elements are downconverted using the same LO-signal with the same phase noise. In an AAS receiver architecture with a distributed local oscillator generation, all antenna element signals are downconverted using different, i.e. uncorrelated or at least partly uncorrelated LO-signals.